1. Field of the Invention
The present invention relates to a manifold for use on storage tanks to allow for dual use of a single port.
2. Related Art
Most liquid storage tanks must have a pressure relieving device installed to protect the tank from being overpressurized. Additionally, many storage tanks are "blanketed", usually with one or more types of gas, for one of several reasons: (1) to prevent the entrance of atmospheric air which contains oxygen, moisture, and other contaminants that could cause internal tank corrosion and degradation of the stored product, (2) to dilute the oxygen content in the tank so that the vapors are below the flammable range (too lean to burn), (3) to enrich the vapors so that they are above the flammable range (too rich to burn), (4) to dilute toxic vapors for safer emission levels, (5) to prevent the collapse of the tank when atmospheric thermal conditions tend to cool the tank walls and cause a drop in internal pressure, and for other reasons.
Some tanks have a vapor recovery system whereby the vapors are piped to a recycling compressor and returned to a liquid state. All of the above described systems require an entry and an exit to the tank. Two ports are usually used in addition to separate ports for pilot sense lines and accessory equipment. Some systems use one port for both the entry and exit.
Some systems use one port on the tank for both the entry of the blanket gas and the outlet for the vapors to the vapor recovery system. Another approach is the use of two ports on a tank for blanketing, one port for blanket gas entry and one port for the control line.
A system previously used by the present inventor provides for a blanket gas entrance and a sensing point through the same nozzle on the tank. There are no provisions for venting the vapors.
Some of the prior systems can perform two of the three needed functions, but have no provisions to exhaust the tank vapors. Other prior systems can accommodate both blanket gas entry and vapor exhaust but the tee is not a good location for sensing tank pressure. Additionally, if a "weight loaded" exhaust valve is used for vapor recovery, with the prior systems it can open prematurely (before the actual set point is reached) because the potential exists for the blanket gas to enter the tee at a much higher pressure than the set pressure of the exhaust valve, thus causing the exhaust valve to open prematurely.
It is an advantage of the present invention that the manifold can accommodate (1) the entry of the blanket gas, (2) the exhaust of the vapors, and (3) sensing point(s) to pick up the true tank pressure, all available in a single unit for mounting on a single port on the tank.